Electric marine propulsion system

ABSTRACT

An AC marine propulsion system which provides a constant continuous rated horsepower availability at the standard rated engine RPM, over the full useful operating range of the propeller. A prime mover diesel engine is operated at its continuous rated speed to provide for maximum fuel efficiency. An alternator connected to the prime mover is driven at a speed which is higher than the standard rated alternator speed, thus producing higher than standard rated voltage and frequency. The combination of prime mover and alternator provides for an optimum system mass to power ratio. An AC motor is coupled through a gearbox to a fixed pitch propeller system. A frequency converter is dispsoed between the alternator and AC motor. Propeller RPM is controlled by changing the frequency of the voltage supplied to the AC motor. Variable shaft RPM at constant horsepower output is supplied to a fixed pitch propeller by selectively varying the frequency of the voltage supplied by the converter from 100% of maximum RPM to 40% of maximum RPM at 100% shaft horsepower.

BACKGROUND OF THE INVENTION

The present invention relates to electric marine propulsion systems.Specifically, an AC electric motor drive is provided for a fixed pitchpropeller wherein full power is available at variable shaft RPM andtorque.

Marine propulsion systems are of two basic types with regard to thestructural design of the propeller. The first basic type is a propellerof "fixed pitch", usually solid cast, wherein the propeller RPM iscontrolled to comply with the demands of the system. Electric motordrives for fixed pitch propellers are described in various references,including U.S. Pat. No. 4,114,555, and in a paper entitled "SCRControlled Electric Propulsion System" by Harry W. O'Brien, Jr.,delivered at the spring meeting of the Society of Naval Architects andMarine Engineers in New Orleans, La. on Apr. 1, 1977. These referencedsystems generate alternating current, and subsequently convert the ACcurrent to DC for driving a DC motor. The prime mover driving thealternator is usually a diesel engine operated at a governed speed,which can produce maximum horsepower upon demand. As such, the primemover is operated at its continuous rated speed to produce, upon demand,its rated horsepower output. These systems typically control motor RPMby varying the applied DC voltage through the use of an SCR controller.As such, the variable DC voltage supplied to the motor determines thepropeller shaft RPM and shaft horsepower, as required. The DC motor isusually coupled to the propeller shaft through a reduction gear box.

The foregoing referenced electric propulsion system provides for controlof propeller shaft RPM over a first and second operating speed range,thus producing constant torque up to 100% of base power and shaft RPM,and a variable reducing torque, at 100% of base power, while operatingat a shaft RPM above base RPM. The horsepower output of the propulsionsystem also varies proportionately with changes in shaft RPM up to 100%of base power, thus providing a variable horsepower, constant torquesystem, up to 100% of base power and shaft RPM. A constant horsepowerreducing torque system is provided once full horsepower output at 100%of base horsepower is attained at the top of the first range ofoperational RPM, wherein horsepower remains constant during operation ofthe vessel, from the bottom to the top of the second range ofoperational RPM. The foregoing electric propulsion system is extremelyexpensive and is inherently burdened with a large mass to power ratio.

Al alternative to the fixed pitch propeller, DC electric drive is thenon-electric controllable pitch propeller drive, usually of precisionmachined fabricated design, which provides the availability of fullengine horsepower at a constant propeller shaft RPM. During operation ofa controllable pitch propulsion system, the engine can maintain its fullhorsepower and RPM while complying with the system's various loaddemands by increasing or decreasing the pitch of the propeller. Theadvantage of the controllable pitch propeller system is a fullymechanical power train, with the ability to maintain 100% fullhorsepower while operating under the various load conditions experiencedduring operation of the vessel.

The controllable pitch propulsion system, however, incursproportionately excessive frictional drag losses when operating underlow pitch, high slip conditions, which diminish the overall propulsiveefficiency of the propeller system. As the pitch of the propeller isreduced while shaft RPM and power remain constant, the horsepower whichis sacrificed to overcome frictional drag losses becomes very large,when compared to the greatly reduced frictional drag loss of a fixedpitch propeller capable of operating at much lower RPM, while stillmaintaining full engine RPM and horsepower output. The present inventioncombines the advantages realized in controllable pitch propulsionsystems with those realized in DC or AC-DC electric propulsion systems,while eliminating their respective disadvantages. The system of theinvention can provide a constant horsepower output to a fixed pitchpropeller over a wide range of shaft RPM to affect a variable torquesystem which substantially reduces propeller frictional drag losseswhile operating at low shaft RPM, full power and high propeller slip.The electric propulsion system of this invention provides substantially100% horsepower over the full useful RPM range of the propeller, whilemaintaining optimum utilization of the available horsepower.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a marine propulsion systemwhich produces a variable torque propeller drive, utilizing constanthorsepower over a wide range of shaft RPM.

It is a more specific object of this invention to provide a totallyalternating current marine propulsion system which drives a propeller,such system having a minimum mass to power ratio compared to otherelectric propulsion systems.

It is a principal object of this invention to provide a variable torquepropulsion drive system, which is capable of producing a more idealbalance between frictional and induced propeller drag losses at highslip ratios, which minimizes total drag losses and enhances thepropulsive efficiency of the system by at least 5% over other types ofpropulsion systems.

These and other objects are provided by the marine propulsion system inaccordance with the invention.

The propulsion system in accordance with the present invention, convertsprime mover internal combustion engine power output into an alternatingcurrent.

Typically, the prime mover is a diesel engine operated at its continuousrated RPM to provide for continuous rated horsepower upon demand. Thisoperating condition for the prime mover produces its maximum efficiencyfor fuel consumption. An alternator is driven by the diesel engine at arotational speed which produces a voltage and frequency greater than therated voltage and frequency for the alternator. Thus, the prime moveroperating at its continuous rated speed, in conjunction with analternator driven so as to produce a voltage and frequency higher thanits rated voltage and frequency, becomes a major system operativeparameter. The generated voltage from the alternator at higher thanstandard voltage and frequency, provides the operative voltage forrunning an AC main propulsion motor.

The higher than standard frequency is converted to any selectedfrequency using frequency conversion apparatus, and applied to an ACmotor. The motor is subsequently coupled by reduction gear means to thepropeller shaft having a fixed pitch propeller attached to its outboardend.

The propulsion system, in accordance with the present invention, makesit possible to achieve a constant engine horsepower and RPM at maximumfuel efficiency, while changing the propeller RPM to attain an improvedpropulsion efficiency. Motor RPM is easily changed by changing theoutput voltage frequency of the conversion apparatus. When it isdesirable to maintain a constant horsepower output at increasedpropeller loads, propeller shaft torque is inversely proportional to theshaft RPM.

Optimum energy conversion is provided with the present invention bymaintaining the prime mover at its continuous rated speed, whilepermitting the alternator to assume an operating RPM, higher than itsstandard rated RPM. As such, the prime mover need not be oversized so asto compensate for the diminished horsepower output often realized, whendriving an alternator at the alternator's standard rated rotationalspeed. Thus, a minimum system mass to power ratio is achieved.

During operation of the propulsion system of this invention, the fixedpitch propeller will be rotated at variable RPM as selected by thefrequency converter. The fixed pitch propeller will, however, absorbless total drag losses at high slip than a conventional controllablepitch propulsion system, and the increased torque at reduced RPM willmore efficiently handle the increases experienced in propeller loads.

DESCRIPTION OF THE FIGURES

FIG. 1 is an overall block diagram of a propulsion system in accordancewith the preferred embodiment.

FIG. 2 is a graph demonstrating theoretical system efficiency versuspropeller torque.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a block diagram of a propulsionsystem in accordance with one embodiment of the invention. Theembodiment depicted in FIG. 1 is a twin screw marine vessel, havingseparate power controls 47, 48 for each propeller 44 and 45. Thepropulsion system provides for a constant horsepower variable torquedrive to each propeller.

The twin screw propulsion system of FIG. 1 includes first and seconddiesel engine prime movers 11 and 12. The diesel engines 11 and 12 arepreferably Model 16 V149T1, General Motor diesel engines, each having acontinuous rated SHP of 1500 at 1800 RPM. The engines 11 and 12 areoperated at a constant speed of 1800 RPM as set by a governor includedwith the engine.

A third smaller size engine 13 is shown which is employed for emergencyshipboard power. The diesel engine 13 will drive alternator 17 toprovide converted shipboard utility power at 60 cycles.

Alternators 15 and 16 may be standard rated at 530 kw., 60 Hz.- 8 pole,240 volts having a continuous rated RPM of 900. Alternator 17 may bestandard rated at 100 kw., but is otherwise the same as alternators 15and 16. Alternators 15, 16 and 17 are driven by the diesel engines 11,12 and 13 at the controlled speed of 1800 RPM, thus producing a higherthan standard frequency of approximately 120 Hz. Additionally thevoltage output of alternators 15, 16 and 17 will double from 240 to 480volts under these input drive conditions. The kw of the alternators 15,16 and 17 will also double from 530 to 1060 kw., and from 100 kw. to 200kw., respectively, but, of course, do not produce an increased currentoutput under these drive conditions.

The foregoing combination of engines 11, 12 and 13 and alternators 15,16 and 17 provides for a maximum energy conversion from diesel power toelectric power at a minimum system mass to power ratio. As such, thediesel engine of this invention may be selected to have a ratedhorsepower which is optimum, thus permitting the engine to be run at itsoptimum speed of 1800 RPM. Alternators 15, 16 and 17 are driven at thisoptimum engine RPM at higher than standard frequency. In the prior artelectric power propulsion systems, the alternators 15, 16 and 17 wouldbe driven at their standard rated speed to produce their standard ratedvoltage and frequency. As such, the prior art devices often impose aninefficiency on the diesel engine prime movers 11, 12 and 13 notincurred by the present invention. Since these prior art systemsgenerate voltage at a standard frequency, the prime mover must always besized so as to make up for the 25-30% loss in horsepower output normallyexperienced by running the prime mover at the alternator's standardrated RPM. When running the alternator at higher than its rated RPM soas to match the standard rated RPM of the prime mover, the prime moverof this invention need not suffer any operating loss, while thepropulsion system is further enhanced by a proportional increase inalternator kw. output. This relationship between prime mover andalternator which achieves a minimum mass to power ratio is describedmore fully in our previous patent application, Ser. No. 191,856, filedFeb. 29, 1980, hereby incorporated by reference.

A common bus 18 combines the power output of alternators 15 and 16. Thecombined alternator outputs of 15 and 16 are, of course, phased togetheras is known to those skilled in the art, such that the 480 volts at 120Hz. is maintained. This three-phase power is to be converted throughfrequency converters 20, 21, 22 and 23 to a range of variable frequencysuitable for driving propulsion motors 28, 29, 31 and 32.

This three-phase power also supplies the main shipboard electricalservice through the interlock system wherein it is first transformed bytransformer 26 to a suitable voltage, and then converted throughfrequency converter 24 to standard 60 Hz. frequency.

Each of the propellers 44 and 45 is connected by shafting to a reductiongear box 41, 42. The propellers are a conventional solid cast, 4-bladedesign of approximately 120 inches diameter, with a 92-inch pitch andhaving a pitch ratio (P/D) of 0.76. The propellers are designed tooperate at the full 1,340 SHP, over a range from 171 to 68 RPM. Theforegoing propeller specifications are of the type which will satisfythe requirements of a typical riverboat towing vessel. The reductiongear box 41 and 42 is a double pinion reduction gear, designed with a10.5 to 1 reduction ratio. The gear box design will handle 100% SHPthrough the full range of 171 down to 68 shaft RPM, with the SHPbecoming proportional to the shaft RPM (decreasing) at shaft speedslower than 68 RPM.

AC motors 28, 29, 31 and 32 are coupled to the input of reduction gearboxes 41 and 42. Coupling members 35, 36, 39 and 40 connect theappropriate motors to the reduction gear boxes 41 and 42. The AC motorsmay be a four-pole design, sized and rated to produce their full powerof 670 horsepower each through an operating range of 1800 RPM down to720 RPM without excessive heating. The foregoing motors, when operatedbelow 720 RPM or 40% of their maximum RPM, will produce an outputhorsepower proportional to the reduced operating speed.

Each of the AC motors 28 and 29 or 31 and 32 is supplied a voltage froma respective frequency converter 20, 21, 22 and 23. Frequency converters20 through 23 receive as an input the higher than standard voltage andfrequency of the alternators 15 and 16. Frequency converters 20, 21, 22and 23 will supply constant voltage at a variable frequency under thecontrol of power controls 47 and 48. Each of the power control panels 47and 48 is linked to the frequency converters 20 through 23 such that thepilot can independently control the RPM of propellers 44 and 45. Controllinks 49 and 50 may be electrical cables which will vary the resistanceof frequency controlling elements of each frequency converter.Alternatively, mechanical linkage may be utilized to vary thosepotentiometers shown in U.S. Pat. No. 3,579,086.

The frequency converters 20 through 23 are of a type known as acyclo-converter, similar to those described in U.S. Pat. No. 3,579,086.The frequency converter employs an oscillator which will reduce theinput frequency in proportion to the oscillator frequency. The frequencyconverter 20 is shown more descriptively in the Patent, herebyincorporated by reference. As such, the control of the propeller isaccomplished by changing the frequency of a voltage applied to the ACmotor.

Frequency converter 24 is a fixed frequency converter, similar to theaforementioned cyclo-converter described in U.S. Pat. No. 3,579,086. Atransformer 26 supplies the frequency converter 24. As such, alternator17 and diesel engine 13 may be driven at a higher than standardfrequency RPM, and the correct 60 cycle voltage will be realized fromthe frequency converter 24. This resulting, transformed 60-cycle voltagemay be distributed throughout the vessel to be utilized as ships servicepower.

Referring to the aforementioned Patent, there is shown the generallayout of a frequency converter for reducing a 120-cycle input voltageto a 60-cycle voltage frequency, and then to a variable frequencyvoltage from 60 Hz. to 0 Hz. Each of the propellers 44 and 45 arecontrollable by the aforesaid pilot house controls 47 and 48 whichcontrol the frequency of the voltage produced from a respectiveconverter.

The performance of the system of FIG. 1 is shown more completely in FIG.2. FIG. 2 demonstrates the system efficiency for motor input voltagefrequencies from 60 to 15 Hz. As is shown to the marine design engineer,a variable torque propulsion system is excellent, if it can provide fullpower at a maneuvering RPM equal to 50% of its maximum RPM. As such, itis seen that in the 60 Hz. to 24 Hz. operating range of this inventionthis design criteria is easily met. The entire system efficiency isshown to vary between 90 and 65% over the operating motor frequency of60 to 24 Hz.

As the motor frequency is varied between 60 Hz. and 24 Hz., thepropeller shaft torque is shown to increase while the motor RPMdecreases. Although shown as a straight linear change, the propellershaft torque will, in reality, show a very slight curve between the 60to 24 Hz. frequency range. However, for all practical purposes,propeller shaft torque may be considered linearly proportional to theRPM and motor frequency.

Over this wide operating range of motor frequency, a constant horsepoweroutput may be applied to the propellers 44, 45. The result of this typeof operation can be more clearly shown in the following comparison ofthe theoretical performance of the present invention over that of aconventional controllable pitch propeller system. As shown in the Tablebelow, the solid cast fixed pitch system produces frictional drag losseswhich rapidly decrease from maximum RPM to 40% of maximum RPM. Althoughthe induced drag losses increase substantially over the same operatingrange, in the embodiment of the present invention, the induced draglosses are more than offset by the reduction in frictional drag losses,plus the accompanying increase in torque. As such, the large increase inpropeller shaft torque at 100% power constitutes a system efficiencyadvantage of the present invention over the controllable pitch propellersystem.

Thus, one can see that the benefits of a controllable pitch propulsionsystem are more efficiently realized with the present invention. Fullhorsepower at increased torque availability is provided over the mostimportant segment of the operating range of the propulsion system.Therefore, optimum vessel speed can be more nearly maintained when anincreased load is realized by the vessel. The availability of the fullAC system horsepower for varying propeller RPM provides a performancepreferable to the controllable pitch propeller propulsion system, andalso preferable to the referenced AC/DC constant torque electricpropulsion system.

Thus, there has been described with respect to one embodiment of theinvention, an AC electric marine propulsion system providing constanthorsepower with variable torque to a solid cast fixed pitch propeller.Those skilled in the art will recognize yet other embodiments describedby the claims which follow.

    __________________________________________________________________________    COMPARISON BETWEEN TWO PROPULSION SYSTEMS OF EQUAL SHAFT                      HORSEPOWER AND PROPELLER SIZE                                                 FIXED PITCH PROPELLER "AC" ELECTRIC SYSTEM, VARIABLE TORQUE                   VERSUS                                                                        CONTROLLABLE PITCH PROPELLER MECHANICAL SYSTEM, CONSTANT TORQUE                      B     C      D        E                         H                      A      Propeller                                                                           Propeller                                                                            Propeller                                                                              Propeller   F     G       Induced                Propeller                                                                            Shaft HP                                                                            RPM at Pitch (Ratio)                                                                          Torque at   Propeller                                                                           Fric. Drag                                                                            Drag (H1)              System at 100%                                                                             100% Power                                                                           at 100% Power                                                                          100% Power  Slip %                                                                              Losses %                                                                              Losses %               __________________________________________________________________________                                                           SHP                    Fixed Pitch Propeller `AC` Electric System                                           1,340 SHP                                                                           Changeable from Max. to 40% of Max. RPM                                              Fixed Pitch Ratio (P/D) Solid Cast                                                      ##STR1##   40% 80%                                                                             25% 10% 10% 25% (H2)           Controllable Pitch Prop. Mechanical System                                           1,340 Constant at Max. RPM Only                                                            Changeable Pitch Ratio (P/D) Con- trollable Pitch                                       ##STR2##   40% 80%                                                                             25% 25% 10% 15%                __________________________________________________________________________                                                           (H3)                    In the foregoing comparison between a `Variable` Torque and a `Constant`      Torque system, the "relative" drag losses shown would normally occur, onl     during conditions such as "maneuvering" operations, or whenever the           propeller is subjected to rapid "slip acceleration", or reversal, as in       switching from full ahead to full astern, and vice versa. Under these         severe (zero to max.), thrust conditions, the Variable Torque system is       capable of faster (thrust), recovery, and hence, greater utilization of       the available shaft horsepower. Consequently, in a towing operation, it       can provide improved "overall" vessel performance.                            (H1) Induced drag, is the "rotational" component imparted to the water by     the action of the propeller. Propellers of comparatively heavy PitchRatio     (P/D), generally produce relatively large `induced` drags when operating      at high slips. On the other hand, propellers of fine PitchRatio (P/D),        generally produce relatively small `induced` drags when operating at high     slips.                                                                        (H2) The relatively large increase in `induced` drag at high propeller        slip and (comparatively), heavy PitchRatio (P/D), is accompanyed by a         similar increase in torque. The increase in `induced` drag is however,        offset by a similar decrease in frictional drag. This even exchange of        drag losses, leaves all of the increased torque available to handle the       increased load (increased slip and thrust), without any further sacrifice     of power, or speed of advance.                                                (H3) The relatively small increase in `induced` drag at high propeller        slip and (comparatively), fine PitchRatio (P/D), is accompanyed by            constant torque. The small increase in `induced` drag is, however, in         addition to the maximum frictional drag. The additional drag losses,          require that a further reduction be made in propeller pitch (controlable)     to satisfy the additional sacrifice of useful power and subsequent lower      speed of advance. Although the `slip` of the Controlable Pitch propeller      may be the same as the Fixed Pitch propeller, the resultant thrust and        speed of advance will be less, due to the fact, that the "speed of the        propeller" (RPM × Pitch), for the CP propeller, has become less tha     that of the Fixed Pitch propeller.                                       

What is claimed is:
 1. A marine propulsion system comprising:a dieselengine for providing a continuous rotational shaft horsepower, saiddiesel engine operated at a speed of maximum efficiency; an alternatorhaving a standard rated frequency output for a standard rated input RPMconnected to be driven by said diesel engine, said alternator standardrated RPM selected to be less than said generated diesel enginecontinuous shaft RPM whereby voltage is generated having a higherfrequency than said rated standard frequency and said diesel engine andalternator produces a power output at a maximum efficiency; a frequencyconverter connected to receive voltage from said alternator, saidfrequency converter providing an alternating output voltage having aselectable frequency; an AC motor connected to receive a voltage fromsaid frequency converter, said AC motor producing a horsepower outputwhich is constant and a torque proportional to the frequency of saidreceived voltage; and gear reduction means for coupling said AC motorand a propeller shaft, whereby said propeller shaft RPM is controlled bysaid frequency converter output voltage frequency, which may be variedby said frequency converter.
 2. The marine propulsion system of claim 1wherein said alternator is selected to have a rated RPM for producing 60Hz. voltage which is less than said engine rotational shaft RPM.
 3. Themarine propulsion system of claim 2 wherein said alternator rated RPM isone half said engine standard rated shaft RPM.
 4. The marine propulsionsystem of claim 1 further comprising a transformer having an inputcoupled to receive said alternator output voltage, and an output coupledto a second frequency converter for converting the voltage received fromsaid alternator to a frequency of 60 Hz.
 5. The marine propulsion systemof claim 1 wherein said alternator produces a voltage having a frequencyof 120 Hz.
 6. A marine propulsion system for independently driving apair of propeller shafts, comprising:first and second diesel engines,each of said diesel engines connected to drive first and secondalternators, whereby a pair of diesel generator sets are provided, saiddiesel engines having a controlled shaft operating speed set to producemaximum shaft horsepower at maximum efficiency, said alternatorsselected to have a standard rated RPM for producing a standard ratedoutput voltage frequency substantially less than said operating speed,whereby a maximum KW power generation efficiency is obtained at avoltage having a frequency higher than said standard rated outputvoltage frequency; a pair of frequency converters connected to each ofsaid alternators for providing a variable frequency output voltage, saidfrequency being selected by an operator of said system to control theRPM of said propeller shafts; an AC motor connected to each of saidalternators, said AC motors providing an output shaft RPM which variesalong with torque in response to a varying input voltage frequency whilemaintaining output shaft horsepower constant; and, a pair of reductiongear transmissions, each coulping a pair of said AC motor shafts to arespective propeller shaft, whereby each of said shafts is driven by apair of motors at an RPM and torque controlled by selecting the voltageoutput frequency of said frequency converters.
 7. The marine propulsionsystem of claim 6 wherein said alternators are driven at an RPM toproduce an output voltage frequency of two to three times said ratedoutput voltage frequency.
 8. The marine propulsion system of claim 6wherein each of said AC motors is selected to provide full horsepower atan RPM range between 1800 and
 720. 9. In a marine propulsion system, amethod for driving a solid cast propeller having a fixed pitchcomprising:coupling an AC motor to said propeller, said AC motorproviding a varying torque at varying RPM to said propeller in responseto a varying input voltage frequency; supplying an AC voltage to said ACmotor having a frequency for selecting said AC motor RPM; andcontrolling the frequency of said AC voltage whereby the RPM of said ACmotor shaft and coupled propeller is controlled.
 10. The method fordriving a propeller according to claim 9 wherein said AC voltage isproduced in accordance with the steps of:driving an alternator having astandard rated RPM which produces 60 Hz voltage with a diesel engine ata speed which produces a maximum diesel engine efficiency whereby saidalternator is driven at an RPM to produce a voltage having a frequencysubstantially higher than 60 Hz.; and, frequency converting saidalternator voltage to a frequency for obtaining a selected RPM.
 11. Themethod according to claim 10 wherein said alternator RPM is selected toproduce a voltage frequency of 120 to 180 Hz.
 12. The method accordingto claim 9 wherein said AC motor is selected to provide full horsepowerbetween an RPM within the range 1800 to
 720. 13. A marine propulsionsystem for providing a variable drive to a constant pitch propellercomprising:a diesel engine driven alternator for providing alternatingcurrent, said diesel engine operated at a speed of a maximum efficiency,and said alternator producing a current having a frequency higher thanits standard rated frequency of operation; a frequency converterconnected to receive said alternator current having a frequency higherthan a standard rated frequency of operation, said converter providingan output voltage at a frequency which is selectable; and an AC motorconnected to receive said frequency converter output, and coupled todrive said propeller, said AC motor providing a variable rotationalspeed to said propeller proportional to a selected voltage frequencyfrom said frequency converter, said AC propeller motor providing aconstant horsepower for each selected frequency.
 14. A marine propulsionsystem of claim 13 wherein said alternator is driven at a speed togenerate voltage and frequency of twice said alternator standard ratedvoltage and frequency.